1. Field of the Invention
The present invention relates, in general, to vibration motors and, more particularly, to a flat-shaped vibration motor having a stable driving characteristic.
2. Description of the Prior Art
One of the important functions in a general communication terminal is a call receiving function. The call receiving function may be performed by indicating a call reception by means of an audible indication, a visible indication or other recognizable indications on a communication terminal in response to a call signal for a switching station as well as closing a talking circuit.
On the other hand, of the recognizable indications, the most popular recognizable indication is to indicate a call reception by means of vibrations because this indication does not cause excessive noise in a crowded place.
As well known to those skilled in the art, the vibrations may be generated by a vibration motor. That is, a small-sized vibration motor is operated and the vibrating force of the operated vibration motor is transmitted to the casing of a communication terminal, so that the casing of the communication is finally vibrated. A user knows of a call reception by recognizing the vibration of the casing.
FIG. 1 is a cross section showing a conventional flat-shaped vibration motor.
The conventional flat-shaped vibration motor comprises a base plate 1 at its bottom. A shaft 2 is vertically mounted in the center of the base case 1. A lower PCB (Printed Circuit Board) 3 is seated on the upper surface of the base case 1, the lower PCB 3 having a circuit through which electric power is supplied. In detail, the lower PCB 3 is mounted in the hollow seating recess, the seating recess being formed on the upper portion of the base case 1. A doughnut magnet 4 is mounted on the upper surface of the lower PCB 3, north poles and south poles being alternately and regularly arranged to have regular intervals along the circle of the doughnut magnet 4. Two brushes 5 are mounted to the lower PCB 3, with the lower ends of the brushes 5 being respectively connected to the input and output terminals of the lower PCB 3, the upper ends of the brushes 5 being positioned higher than the upper surface of the magnet 4, the brushes 5 being spaced apart from each other by a certain angle. An upper PCB (Printed Circuit Board) 7 is fitted around the lower portion of a bearing 7a, the upper PCB 7 having a shape unbalanced in weight, the bearing being fitted around the upper portion of the shaft 2. A commutator 8 consisting of multiple segments is positioned beneath the upper PCB 7, around the bearing 7a and in contact with the upper ends of the brushes 5. A coil unit 9 is mounted on the unprinted portion of the upper PCB 7, the number of the elemental armature coils of the coil unit 9 being determined according to a driving system of the vibration motor. For example, one armature coil, two armature coils and three armature coils may be respectively employed in the vibration motors of a single phase driving system, a two-phase driving system and a three-phase driving system. When two or more armature coils are employed in the motor, they are spaced apart from each other by a certain angle. The portion, on which the armature coil or coils are not mounted, is provided with insulation 7b so as to insulate the armature coils 9 from each other and increase the unbalance in weight. The insulation 7b may be integrated with the upper PCB 7 into a single body through an insert injection process. A cover 6 is put on the base plate 1 and holds the upper end of the shaft 2.
In brief, the conventional vibration motor consists of a stator and a rotor. The stator comprises the base plate 1, the shaft 2, the lower PCB 3, the magnet 4, the brushes 5 and cover 6, while the rotor comprises the upper PCB 7, the commutator 8 and the coil unit 9.
The conventional vibration motor is operated as described below.
When external power is supplied through the lower PCB 3, the power is introduced to the commutator 8 through the brush 5. After that, the power is supplied from the commutator 8 to the armature coil or coils 8 through the printed circuit of the upper PCB 7. As a result, the electromagnetic force is generated by an interaction between magnetic fluxes of the armature coil or coils 8 and the magnet 4, thereby rotating the rotor. In detail, the electromagnetic force is converted into an unbalanced rotating force because the rotor of the motor is unbalanced in weight. The unbalanced force is transmitted to the base plate 1 and the cover 6 through the shaft 2, so that a user recognizes vibrations of the motor. The three-phase driving system has been employed so far, but simply structured single phase and two-phase driving systems begin to be employed as the systems are developed. However, the single phase and two-phase driving systems are problematic in that the driving of the motor becomes unstable compared with the two-phase driving system because the continuous driving force is not maintained.
That is, in the single driving system, when the rotor is rotated by the electromagnetic force generated by the interaction between magnetic fluxes of the coil and the magnet, there occurs a dead point in which the driving force is eliminated upon changes of polarities due to directional change of current in the coil.
In order to solve such a problem, for the single driving system, there are provided a cogging generating device wherein a proper cogging is generated in the dead point, thus allowing a continuous driving to be performed. However, since the cogging generating device is of minute size, it is very difficult to mount on a precise position. Therefore, this renders productivity of the motor to be reduced, and generates position deviations in manufactured motors, thereby causing torque deviations in the manufactured motors.
On the other hand, in regard to the two-phase driving system, power should be introduced to at least one of the two-phase armature coils 9 and, to this end, a pair of brushes connected to the commutator 9 have a certain interval angle therebetween. That is, when the number of the commutator segments is "n", the interval angle between a brush and an adjacent brush should be 360.degree..multidot.2/n. For example, the interval angle between a brush and an adjacent brush should be 180.degree. as shown in FIG. 2 when the number of the commutator segments is four, and 90.degree. as shown in FIG. 3 when the number of the commutator segments is eight.
However, when an actual interval angle between two brushes is less or greater than the above described standard angle, the dead point occurs because the brushes 5 are respectively connected to the armature coils 9 having different phases.
Therefore, in a two-phase driving system, the interval angle between the brushes, which contact the commutator 8, should be kept correct. However, since it is almost impossible to connect two brushes 5 to the lower PCB 3 at an exact interval angle, mechanical errors occur, thereby causing bad driving performance and defective in manufactured products.